A majority of stroke survivors have residual gait deficits. Restoration of walking ability is a major goal of rehabilitation. Gait retraining can improve walking speed and activity even in stroke survivors who are discharged from rehabilitation and assumed to have reached a plateau in recovery. However, while there is agreement that gait rehabilitation improves post-stroke walking function, consensus is lacking on which specific interventions, strategies, or dosing regimens are most efficacious. One factor contributing to the lack of consensus in rehabilitation literature is that the dose-response time courses and neuroplasticity mechanisms underlying gait rehabilitation have not been systematically studied. The goal of this proposal is to address the gaps in our understanding of neural correlates of clinical post-stroke gait rehabilitation, the knowledge of which would inform the development of more effective rehabilitation protocols. To meet this goal, we propose to assess time courses of cortical and spinal neurophysiologic correlates underlying a novel and effective post- stroke gait rehabilitation paradigm combining fast treadmill training and functiona electrical stimulation (FastFES). The innovation of the proposed research is the concurrent evaluation of biomechanical (gait kinetics and kinematics), neuroplasticity (cortical and spinal), and functional (gait speed and endurance) processes during 18 sessions of FastFES gait retraining. These multi-modal evaluations will be performed in 2 groups of stroke survivors (FastFES versus dose-matched control). The aims of the research plan for this proposal are to (1) determine whether 18 sessions of the FastFES gait treatment produce greater changes in cortical and spinal excitability versus a control treatment in individuals post-stroke, (2) compare the time course of changes in corticospinal excitability, gait biomechanics, and walking function during FastFES gait retraining in individuals post-stroke, and (3) determine relationships among changes in corticospinal excitability and gait biomechanics after gait retraining. Insights into tie courses and neural correlates underlying clinical post- stroke gait rehabilitation will impact clinical practice by aiding with the development of effective, individualized strategies based on neuro-biological principles to maximize the benefits of gait rehabilitation. The Principal Investigator's doctoral and post-doctoral training has imparted her expertise in gait biomechanics and gait rehabilitation. The Principal Investigator's long-term goal is to conduct high-impact research at the juncture of biomechanics, neuroscience, and gait rehabilitation that profoundly impacts walking function and quality of life of individuals with neurological disability This Mentored Research Scientist Development Award will enable the candidate to gain additional training and proficiency in the use of TMS as a tool for evaluating corticomotor control of ankle muscles, and in the electrophysiological evaluation of spinal circuitry controlling the ankle muscles. The mentoring team comprises established scientists with expertise in neuroplasticity mechanisms underlying post-stroke recovery and rehabilitation.